Retaining rings are cylindrical steel components mounted to either end of an electrical generator rotor. At the rotor ends, copper windings exit slotted portions on the rotor body and form unsupported arcs to the opposite side of the rotor's magnetic poles before they re-enter other slotted portions These arcs are known as the endturns of the windings. The retaining rings contain and support these end-turns against rotational forces.
Other components made of similar materials and used in similar rotating applications include zone cooling baffles and retaining rings on exciters. Because of their similarity in function, failure mechanisms, and necessary care, all such components are included in the general description of a retaining ring.
Retaining rings typically suffer from flaws and stress corrosion. Flaws are unintentional imperfections formed during construction of the ring Examples of flaws include cracks, structural variations, pits, and other imperfections. Stress corrosion occurs when forces, pressures, stresses, and deleterious environments are imparted to the ring during its use. Examples of stress corrosion are staining, pitting, and cracking.
Retaining rings have traditionally been made as a single piece through seamless forging. This is done in order to provide the necessary combination of strength, toughness, homogeneity, and freedom from flaws required in the use and application of the retaining rings.
Retaining rings are usually assembled to a rotor with a shrink fit at one end of the ring. Additionally, an end plate is normally assembled to the other end of the retaining ring to provide access for balance weights and to stiffen the retaining ring in order to minimize non-cylindrical deformation from non-uniform winding loads. This end plate is also typically assembled to the retaining ring with a shrink fit.
The shrink fits at both ends of the retaining ring place the material at and near these shrink fits under maximum stresses when the rotor is at standstill or slow speed operation. Furthermore, even at synchronous speed, the overall maximum stresses do not dramatically change. As a result, stress corrosion can occur at any time.
Detection of flaws and stress corrosion in a retaining ring is extremely important because undetected defects may result in sudden failures causing extensive damage to the generator, the surrounding equipment, and personnel. One method for detecting flaws and stress corrosion in a retaining ring involves eddy current inspection of the inner surface of the ring.
Eddy current inspection is a non-destructive procedure used to detect flaws and stress corrosion in electrically conductive materials. This method involves placing an eddy current probe, comprising a coil, near the electrically conductive material. The coil sets up a magnetic field and induces eddy currents in the material. Defects in the material alter the eddy current flow and change the impedance of the coil. As a result, flaws and stress corrosion may be detected by moving the eddy current probe along the material and detecting changes of impedance of the coil.
Some previous scanning devices for eddy current inspection of retaining rings have not been automated. Because of the sensitivity of the probe and the lack of control in manipulation of the probe, these devices have suffered from probe lift off and probe wobble. As a result, these devices do not provide highly accurate detection of flaws and stress corrosion.
Other scanning devices have however been automated. One such device involves supporting an eddy current probe inside the ring for axial and rotational movement of the probe along the inner surface of the ring. This is accomplished by mounting two tripod shaped supports inside the ring. A drive rod is rotatably supported on the supports and an arm is fixed to the drive rod. A floor mounted drive assembly located away from the ring provides automated axial and rotational movement of the drive rod. Since the probe is mounted to the arm, it scans the inner surface for flaws and stress corrosion in a predetermined path between the support rods.
This prior art scanning device suffers from a number of limitations. First, since the supports are mounted inside the ring, the entire inner surface of the ring can not be inspected. Second, alignment of the drive rod along the axis of the ring is difficult to accomplish because horizontal adjustment of the tripod shaped supports necessarily affects vertical alignment, and visa versa. Furthermore, once the tripod shaped supports are aligned, alignment of the floor mounted drive assembly with the rotation axis of the supports is difficult to achieve and time consuming.
Thus, there is a need for a scanning device and method for automated eddy current inspection of retaining rings providing high reliability and the capability of time efficient inspection of the entire inner surface of the ring.